Yarl’s Wood in Bedfordshire, the only centre designed for the detention of women.
Photograph: Sean Smith for the Guardian

How many people are locked up pending deportation?

Immigrants can be detained at any time. The Home Office casts its net widely: anyone deemed not to have the right to reside in the UK can be detained and deported. Those who do not have legal representation, who do not speak English and who are newly arrived in the UK are least able to challenge a Home Office decision to detain them.

More than 27,000 people were detained in 2017, according to the most recent figures. Detention is now a significant part of the UK’s immigration enforcement efforts, but locking up immigrants without a time limit is a relatively recent phenomenon.

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How does it happen?

Some people are detained as soon as they arrive by Home Office officials stationed at airports and ports. Others can be detained after living in Britain for many years when they try to renew work, family or study visas.

The Guardian survey found 15% were detained at a Home Office reporting centre, where immigrants must attend regular appointments while their applications are processed. They can find themselves starting the day queuing to see a bureaucrat and ending the day in a small cell with a stranger.

Others were apprehended during dawn raids at their home addresses, after rough sleeping, found working illegally or while making applications for leave to remain.

The 2007 UK Borders Act introduced “automatic deportation” for some ex-offenders, which has been an important factor in the expansion of the detention population. Guardian research found more than half of respondents were detained at the end of a prison sentence.

Most are exclusively for men, while Yarl’s Wood in Bedfordshire is the only centre designed to hold women.

Foreign national offenders can also be detained in mainstream prisons, typically after serving their criminal sentence. In 2017, 1,691 people were held under immigration powers in prisons.

Less than a quarter of those held in prison have access to legal representation, according to research by Bail for Immigration Detainees, a charity which assists with detainees’ bail applications.

A spokesperson from BiD said: “If any British citizen, anywhere in the world, was kept – without warning – in prison after they had finished serving a criminal sentence there would, rightly, be uproar. Yet the British government routinely holds foreign nationals in exactly that manner.”

Who are the detainees?

The nationality of those held in detention changes over time, depending on global migration flows. The Guardian survey found Nigeria and Algeria were most commonly represented among our responses, while Home Office open data for the second quarter of 2018 showed that South Asian countries made up the largest proportion of detainees.

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Detainees are overwhelmingly male - women made up just 15% of the total detention population in 2017.

The government announced it would end indefinite detention of immigrant families with children in 2010, while detaining unaccompanied children for more than a day was banned in 2014. As a result, the number of children in the system fell from 240 at the start of 2010 to just 22 in the most recent Home Office figures.

However, unaccompanied children may still be detained during criminal cases and escorted during returns. Families are still detained together in “exceptional circumstances”. The Guardian survey uncovered multiple examples of children being detained in adult facilities.

How long do they stay locked up?

The UK is the only country in Europe with no statutory time limit on detention. While most are held for days or weeks, the Guardian survey uncovered several cases where a detainee was held in excess of two years.

Indefinite detention has been a key criticism of UK immigration policy. In 2015, the first ever parliamentary inquiry called for a 28-day time limit on detention. The Guardian survey found the vast majority of detainees are not told how long they will be held for or when they will be deported.

The Shaw report found that more than half of detainees were ultimately released back into the community, posing questions about the use of taxpayers’ money to pursue lengthy periods of detention.

How much does it all cost?

The Home Office has paid a handful of private contractors hundreds of millions of pounds to run the UK’s immigration removal centres, but no one knows for certain just how profitable the industry is.

The Home Office’s annual report and accounts for 2017-18 states detention costs of £108 million in the year ending 31 March 2018, while the Shaw report says it costs £85.92 per day to hold someone in detention.

Commercial confidentiality agreements mean the Home Office and outsourcing companies are not obliged to publish detailed financial information about immigration detention centres in the UK.

Earlier this year Mitie won what is believed to be the largest immigration detention contract ever awarded, valued at more than half a billion pounds. The contract will cover a range of services and it is not known how much of this is for management of removal centres.

The profitability of detention facilities has proved to be a contentious issue for the contractors.

A Guardian investigation last year pointed to a 20.7% profit margin at G4S-owned Brook House in 2016, while at Tinsley House, the margin was 41.5%.